Added Ctrl+B as a shortcut for toggling a breakpoint on the current line...

Addresses issue #168.

I made the changes directly in devtools.js. I'm not sure what is happening with the update-ui branch, but it appears to represent what is out on the node repositories as the latest version (0.2.0beta4). All of the individual files from the master branch have been collapsed down to just devtools.js for some reason… Hopefully this pull request makes sense, please let me know if there is something I should be doing differently.